A technique of forming a thin film on a glass substrate using a chemical vapor deposition method (hereinafter referred to as a “CVD method”) is well known. The thin films to be formed are of various kinds and one of them is a silicon nitride (SiN) film. The silicon nitride thin film has been used widely as an insulating film in the field of semiconductors. In addition, since the silicon nitride thin film has a dense structure, it also is used as a barrier to the diffusion of various ions such as sodium, silver, etc., for example, as an acid-proof mask. A glass substrate with a silicon nitride thin film has high transmittance in the visible light region and thus is suitable for use for buildings, vehicles, or display substrates.
As a method for forming a silicon nitride thin film is known a method of depositing a film by an atmospheric CVD method using monosilane (SiH4) and ammonia (NH3). In the conventional film depositing method, however, the concentration of monosilane contained in a gaseous raw material was relatively low, for example, 0.1 mol % or lower. The silicon nitride thin film has high tension therein and may peel off a glass substrate in some cases, which has been a problem. As a means for solving this problem is known a technique of reducing the tension by including oxygen in a thin film to form a silicon oxynitride (SiON) film. For example, JP10(1998)-309777A discloses a technique of depositing a thin film containing silicon nitride and silicon oxynitride as its main components on the surface of a glass substrate by the CVD method.
Furthermore, for instance, JP2001-100811A discloses a method that takes into consideration the passivation function of a silicon nitride thin film, in order to form a perfect silicon nitride thin film, i.e. a silicon nitride thin film containing no impurities, and the flow ratio (SiH4/NH3) between monosilane and ammonia contained in the gaseous raw material is set at a low ratio, particularly, about 0.086 in a plasma-enhanced CVD method.
However, the film growth rate of silicon nitride and silicon oxynitride by the conventional CVD method is low, particularly about a few nm/s although it also depends on the film forming apparatus. In particular, when the above-mentioned thin film is deposited on the surface of a glass ribbon which floats on molten tin in a float bath in the process of producing float glass by the CVD method (hereinafter, this formation method is referred to as an “on-line CVD method”), it was difficult for the thin film to grow up to a thickness enough for its characteristics to be exhibited fully at a conventional film growth rate. In the on-line CVD method, it is conceived that when a thin film containing silicon nitride and silicon oxynitride as its main components is formed, a film growth rate of at least about 8 nm/s is required to make the thin film grow so that it has a thickness allowing its characteristics to be exhibited fully, although it also depends on the moving rate of the glass ribbon.